How I used the brake controller---
---I wanted the tow vehicle brake pedal feel to be the same, and the braking effort the same, with the same foot pressure on the pedal. I didn't want the trailer to pull back when I braked, and I didn't want to have the tow vehicle to supply more braking due to the weight of the trailer. The brake controller has an adjustment to give this feel.
---Some want the trailer to lead the braking. They want it to pull back on the tow vehicle. That is a real problem on downhill curves. Touch the brakes, the trailer pulls back, and it tends to straighten the tow while you need to smoothly continue around the curve. Also, I do not want to overheat the trailer brakes. Some want the trailer to hold the combination back on a downhill, and that can easily overheat, maybe burn out, the trailer brakes. I like smooth, no drama braking.
---Downshift the tow vehicle on long downgrades. Let the engine slow the rig to almost the speed you feel is safe for your situation (which may be less than the speed limit). Use the brakes intermittently to get down to that safe speed. Get off the brakes, let them cool, then get back on the brakes as needed. Some will say, "brakes are cheaper than a transmission; use your brakes." OK until the brakes get overheated and you crash. Locally we've had two fatality crashes from overheated brake fluid. The brakes get real hot, the old moisture-laden fluid flashes to steam, the pedal goes to the floor, and the vehicle is a runaway. Brake fluid is designed to disperse moisture in it so the inevitable moisture that gets into the system doesn't pool in the calipers and corrode them. Get the fluid renewed periodically.